(1) Field of the Invention
The present invention relates to a nonwoven sheet composed of filaments of a polyester group and a method for producing these sheets. More particularly, the present invention relates to a formable nonwoven sheet having a superior forming property and a good utilizability.
(2) Description of the Prior Art
Nonwoven fabrics have been widely used in place of knitted cloth or woven cloth, but in the most cases, the nonwoven sheet is used as a sheet per se. The nonwoven sheet, especially the nonwoven sheet made by a spun bond system has an air permeability, water permeability, and cushioning property. Therefore, if it is possible to use this nonwoven sheet as a forming material, new applications impossible to achieve by a conventional forming material can be developed.
Where the nonwoven sheet is used as the forming material, the nonwoven sheet should have a superior forming processability and a superior shape retaining property. The superior forming processability makes it possible to produce a formed part or a formed pieces having high convex portions and deep concave portions and/or a complicated shape, in a broad range of forming temperatures. Because of the superior shape retaining property, the formed part is not easily deformed by an external force and the shape of the formed part is not shrunk or deformed by heat. In the description hereafter, a combination of the forming processability and the shape retaining property is referred to as the forming property.
Further, the formed part made of the nonwoven sheet should have a good utilizability. This good utilizability is divided into physical properties and properties during use. For the physical properties, the abrasion resistance of a surface of the nonwoven sheet should be excellent, with little occurrence of fuzz, and the nonwoven sheet should have an adequate air permeability and water permeability. In all applications, the formable nonwoven sheet should have these good physical properties. With regard to the properties during use, the values of the properties during use depend on the applications for which the formable nonwoven sheet is used. For example, when the formable nonwoven sheet is used as a wrapping material and letters and/or marks are printed on a surface of the formed part made of the formable nonwoven sheet, the formable nonwoven sheet must have a smooth surface and a good printability, so that minute lettering or marks can be clearly printed on the surface thereof. If the formed part of the formable nonwoven sheet is used as a core material, for example, a member holding the shape of a shoe and arranged between a surface leather and an inside lining, the formable nonwoven sheet must have a good flexural endurance property, i.e. a property that after the formed part is bent by an external force, the shape of formed part can speedily recover its original shape by removing the external force. At the present time, the formable nonwoven sheet having the above-mentioned properties are not available in the market.
Japanese Unexamined Patent Publication (Kokai) No. 51-40475 discloses a method for improving the forming properties by partially cutting filaments by a needle punching operation. But when a deep draw forming is performed or a formed part having a complicated shape is produced by using this nonwoven sheet, irregular slippage between the filaments occurs, and a thickness irregularity caused by the irregular slippage of the filaments may occur in the nonwoven sheet. Further, the formed part formed by using this nonwoven sheet may be deformed, so that the shape retaining property becomes poor.
U.S. Pat. No. 3,523,149 and U.S. Pat. No. 3,847,729 discloses that a nonwoven sheet made of undrawn filaments was used as the forming material on the basis that a known undrawn filament has a large breaking elongation and shrinkable property. However, this nonwoven sheet can be only used in specific fields such as a vacuum forming material formed by using a mass volume of an adhesive and by laminating with a polymer foil, and cannot be used as a general formable nonwoven sheet. Further, this nonwoven sheet is easily deteriorated by heat, and thus a temperature used in a forming process must be kept at a low level. Accordingly, a formed part produced by using this nonwoven sheet has a poor heat setting property and is easily deformed by heat.
The same applicant as that of the present application proposed a method for stretch-setting a nonwoven sheet under a dry heating condition, using a nonwoven sheet composed of undrawn filaments as a forming material (see Japanese Unexamined Patent Publication (Kokai) No. 60-199961 and Corresponding U.S. Pat. No. 4,578,307). This nonwoven sheet has an excellent forming processability, because it can be easily elongated and deformed at a high temperature. However, since filaments constituting this nonwoven sheet are only interlaced in a partial-heat-press-bonding portion of the nonwoven sheet and bonding of filaments in an area between adjacent partial-heat-press-bonding portions is weak, and filaments in this area are not fixed, a shape retaining property of a formed part made of this nonwoven sheet is poor and the physical properties of this nonwoven sheet are not good. Further, since this nonwoven sheet has a number of the partial-heat-press-bonding portions, a surface of this nonwoven sheet is not smooth and the printability of this nonwoven sheet is not good.
The same applicant as that of the present application further proposed a nonwoven sheet capable of being used in the forming process by heat-setting a nonwoven sheet composed of undrawn filaments in a preset ratio of shrinkage under a dry heating condition, in Japanese Unexamined Patent Publication (Kokai) No. 60-194159. Though the forming processability of this nonwoven sheet is good, this nonwoven sheet has a poor shape retaining property of a formed part made of this nonwoven sheet, and in physical properties and a flexural endurance property in use. Therefore, this nonwoven sheet cannot be used for a core material.
Under the above-mentioned background, we carried out research with a view to eliminating the problems occurring when the known nonwoven sheets are used as the forming material, and as a result, found that a formable nonwoven sheet having superior forming properties and good utilizability can be obtained by heat treating a nonwoven sheet composed of undrawn filaments of a polyester group, under specific conditions.